Pinging

Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models

Created: 2005 October 04

Updated: 2005 October 04

Contributed by Patrick Hayes on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io).

First lets understand what a ping is. The compressed air/fuel mixture in your cylinder does not explode. Rather, it burns at a very fast rate. The flame starts at the tip of the spark plug and progresses outward, like ripples in a pond, until it reaches the cylinder walls and consumes all of the mixture. This progressive flame can be measured and seen. A famous Dr. Diesel put glass windows into cylinder heads about 110 years ago to confirm this. The ripples of pressure ahead of the flame wave cause the mixture to get a little more compressed than just the cylinder compression. Increase it enough, and the remaining mixture spontaneously and completely explodes by adiabatic pressure, ahead of the intended flame front. This can be heard as a ping. Generates lots of instant heat and can melt aluminum. Any of that in your motor? It creates a snowball effect. The pinging makes the engine hotter. The engine pre-heats the air/fuel mixture a little more before it is sparked. Once sparked, the the pre-heated mixture burns more rapidly than the engineer intended it to. It pings and generates more heat again.

The ignition timing curve is designed to allow the flame front to propagate smoothly so that the piston is already starting down and relieving pressure as the flame reaches maximum internal pressure. That's fine on the drawing board. You can do a lot of things to screw up the design.

The progress of the flame is also partially controlled by the nature of the fuel. Premium or Hi Test fuel is designed to burn more slowly so that the flame front is not effected quite so much by high compression motors. Contrary to popular opinion, regular fuel burns faster than premium fuel.

Are you burning regular fuel? Don't do it.

Realize that the ignition timing was designed in the 60's when we could buy 96 octane fuel. Now the best we have is 91. So, the designed ignition timing is too far advanced for the modern fuel. Try backing off the timing by 1° or 2°. You may sacrifice a little performance, but you have to adapt your old motor to the modern fuel reality.

Probably the worst condition for pinging is to contaminate the air/fuel mixture with oil. Minuscule amounts of oil. Far less than you could measure or see burn as smoke. Guzzis have no seals on the valve guides. After a period of wear, they start to bleed some oil past the intake valve stem on suction. Suddenly, your expensive 91 Premium fuel is probably burning at an equivalent of 84 cheapo regular. Ping. When was the last time you did a complete valve job and exchanged the guides and/or valves. My bet is that 40K miles is about the life limit with about the last half of that life passing excess oil.